I think to use one ad9684 adc in my board and my question is if I don't connect SYNC differential pins(left open), is this a problem for ADC to run properly
The SYNC± input is a source synchronous system reference signal that enables multichip synchronization. The input clock divider, DDCs, and signal monitor block LVDS output link can be synchronized using the SYNC± input.
As you note, if you are only using a single AD9684 device the multichip SYNC± feature is really not applicable, and by default this SYNC± feature is disabled on power-up by the default setting in Register 0x120. In this default mode it is recommended to leave the unused SYNC± input pins open/floating.
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